Perfect Fit Diesel Compression Test Adapt Adapter uses with STAR Diesel Compression Tester TU-15. Compatible with GM: Duramax 6.6L Turbo Diesel (2005-present). Also works on a Duramax LML engine. Perfect Design M10-1.25 thread adapter with quick disconnect screws into the head of the engine in the glow plug hole. Manufactured to specific thread tolerance standards. This compression test adapter perfectly fits for the glow plug hole on the engine head, so you don't have to worry about the wrong size that will affect the result of the measurement. Superior Material .